The present invention generally relates to a wireless communication technique, and more particularly, to an apparatus and a method for transmitting signals to one or more users located in a prescribed geographical area.
In the field of wireless communications, various studies are made of the next-generation communication schemes, and researches are also conducted on these communication schemes. In particular, the adaptive array antenna (AAA) technique is one of the technologies that is drawing attention because of its possibility of increasing the capacity for number of subscribers.
An adaptive array antenna includes multiple antenna elements (such as dipole antennas) arranged in an array. By adaptively controlling the weight for each of the antenna elements, the antenna gain can be increased in a specific direction. For example, the antenna gain is increased in a direction to which a signal is forwarded to a counterpart communicating node, while the antenna gain is reduced in the other directions. This arrangement allows efficient signal transmission under reduced interference, and leads to increased capacity for subscribers to be accommodated in a cell or a sector. Such an adaptive control technique is disclosed in, for example, A. Harada, S. Tanaka, M. Sawahashi, and F. Adachi, “Performance of Adaptive Antenna Array Diversity Transmitter for W-CDMA Forward Link”, Proc. PIMRC 99, pp. 1134-1138, Osaka, Japan, September 1999.
Explanation is made of adaptive control in a sector as a geographical area below, and the same applies to adaptive control in a cell.
In a conventional mobile communications system (e.g., a W-CDMA mobile communications system), the downlink includes common channels and dedicated channels. On the dedicated channels, signal transmission is carried out making use of a directional beam generated by the above-described adaptive array antenna, while adapting the beam pattern for each mobile terminal. On the other hand, signals containing information in common for all the users belonging to the sector are transmitted on the common channels. Accordingly, signal transmission using a beam with strong directivity covering only a part of the sector (referred to as “directional beam transmission”) is not implemented. Rather, signals are transmitted using a wide beam pattern covering the entire sector (which is referred to as “omnidirectional sector transmission”).
FIG. 1 schematically illustrates a part of the conventional signal transmission apparatus. The signal transmission apparatus 100 has multiple antenna elements 102, 104 and 106 arranged close to each other. Power amplifiers 108, 110, and 112 are provided to the antenna elements 102, 104, and 106, respectively. Dedicated channels appropriately weighted by the associated weighting adjustors 114, 116, and 118 are connected to the inputs of the power amplifiers 108, 110, and 112, respectively. It should be noted that a composite signal that is a composite of the common channel and the weighted dedicated channel is input to the power amplifier 108 from the signal synthesizer 120.
In operation, signals on the dedicated channels are multiplied by appropriate weighing factors (w1-wN) at the weighting adjustors 114-118, amplified by the power amplifiers 108-112, and transmitted from the antenna elements 102-106, respectively, using directional beams covering only a part of the sector. The common channel is connected to the power amplifier 108 via the signal synthesizer 120, and the signal is transmitted from the antenna element 102. The other antenna elements 104 and 106 are not used for the common channel. The antenna element 102 is an omnidirectional antenna, and the signal from the common channel is transmitted using a wide beam pattern covering the entire sector (i.e., by omnidirectional sector transmission). Consequently, the common channel information is transmitted simultaneously to all the users in the sector.
Since the signal on the common channel has to be transmitted from the antenna element 102 to the overall sector, the power amplifier 108 provided to this antenna element 102 requires a greater ability of power amplification, as compared with the other power amplifiers. Assuming that 20% of transmit power is assigned to the common control channel, and that directional control is carried out for the dedicated channels using ten antenna elements evenly, then the power amplifier 108 outputs 28% of transmit power, while each of the other power amplifiers 110-112 outputs 8% of transmit power. Thus, the power amplifier 108 used for common channel signaling requires a higher amplification ability over the other power amplifiers 110-112.
However, such a power amplifier capable of outputting a high power signal occupies a lot of space, which goes against the demand for reducing the size of the signal transmission apparatus. In general, a spare power amplifier is prepared from the viewpoint of safety and maintenance of the system. The spare high power amplifier is also a large and high-power amplifier. This is also against the demand for miniaturization. This drawback becomes conspicuous as the power required to transmit the common channel (that is, the portion of the entire transmit power) increases. In addition, as the power consumed in omnidirectional sector transmission on the common channel increases, the transmit power assigned to the dedicated channels decreases. This limits the sector capacity.
To overcome this drawback, a technique for transmitting the common channel to the overall sector making use of multiple antenna elements forming the array antenna is proposed. See, for example, Ihara, et al., “Efficient Common Channel Transmission Employing Multi-Beam Antennas for Adaptive Antenna Array Transmit Diversity in W-CDMA Forward Link”. With this technique, the common channel information can be transmitted over the sector via multiple antenna elements by appropriately controlling the weighting on the antenna elements. This arrangement allows an RF circuit (such as a large-size power amplifier 108) dedicated to common channel transmission to be removed.
However, the fundamental idea of transmitting the common channel across the sector is the same, even with the technique disclosed in the latter publication. The total power consumption is the same in principle, regardless of whether using a single antennal element or multiple antenna elements. Accordingly, if the power required to transmit the common channel is increased, concern about decrease of transmit power assigned to the dedicated channels still remains.